1. Field of the Invention
The present invention relates to a laser welding structure and a laser welding method.
2. Description of the Related Art
A laser welding using a laser light has an advantage that a welding process can be completed for several milliseconds. Copper and copper alloy have high reflectance with respect to the laser light. Thus, copper and copper alloy are difficult to absorb energy of the laser light. In addition, copper and copper alloy have high thermal conductivity. Thus, a portion melted by an irradiation of the laser light is prone to recrystallize and a crack may generate at the recrystallization.
U.S. Pat. No. 6,221,505 (corresponding to JP-A-11-104865) discloses a laser welding method in which a first member and a second member are overlapped each other and end portions of the first member and the second member are melted. The laser welding method is premised on welding an object having a cylindrical shape. Thus, it is difficult to apply the above-described laser welding method to a welding of conductive members and leads of a semiconductor device arranged at small intervals. In addition, the above-described laser welding method is not considered for using copper or copper alloy having a high laser reflectance.
JP-A-2007-54854 discloses a welding method of copper and copper alloy using electron beam instead of a laser light. When a welding object is irradiated with electron beam, there is a possibility that electric current flows in the welding object. Thus, in a case where leads of a semiconductor device are welded with conductive members using the electron beam, an element of the semiconductor device may be damaged. In addition, when a welding is performed using the electron beam, a welded portion is required to be disposed in a vacuum state. Thus, a welding equipment may be large and complicated.
JP-A-11-5182 discloses a laser welding method in which a welding object made of copper is covered with a plating layer and end of the welding object is cut on a slant. In such a method, the end of the welding object is required to be cut on a slant in a thickness direction of the welding object. Thus, a productivity may be reduced.
JP-A-2007-144436 discloses a laser welding method in which a welding object is formed into a thin plate shape or a welding object is bent, so that the welding object has a high elasticity. In such a case, a productivity may be reduced because an end of the welding object is required to be formed into the thin plate shape or the welding object is required to be bent.
In a laser welding method disclosed in JP-A-7-214369, a first plated metal member is disposed on a second plated metal member. A laser light is irradiated from a surface-side of the first member opposite to the second member so as to form a welding part extending from the surface of the first member to an inside of the second member over a boundary between the first member and the second member. Thereby, the first member is joined with the second member.
In such a case, a plating formed on the first member is made of a material having a high laser absorption rate compared with a material of the plating formed on the second member. Thereby, a laser energy efficiency can be improved and an explosion of a melted portion irradiated with the laser light can be reduced.
However, there is a possibility that the plating formed on the first member has a melting point lower than a melting point of the plating formed on the second member. In a case where the first member is thick as a lead of an integrated circuit package, even if the plating having a high laser absorption rate is formed on the first member, a high energy is required for melting the plating. Thus, an explosion may occur.